Prometheus is a open source monitoring system that includes built-in and active scraping, storing, querying, graphing, and alerting based on time series data.

Architecture: Prometheus servers run independently of each other and only rely on their local storage for their core functionality: scraping, rule processing, and alerting.

Data model: Most important aspects is its multi-dimensional data model (OpenTSDB) along with the accompanying query language. This allows easy filtering, grouping, and matching by these labels via in the query language.

Scalable: Hundreds of thousands of samples per second. Millions of time series. On a single monitoring server (running many servers is easy too).

Storage: Prometheus own database optimized for time series data that can allow work with millions of time series.

Alerting: Separated into two parts. Alerting rules in Prometheus servers send alerts to an Alertmanager. The Alertmanager then manages those alerts, including silencing, inhibition, aggregation and sending out notifications via methods such as email, PagerDuty and Slack.